DOI: 10.1002/cctc.200900246 Mechanism of CO Disproportionation on Reduced Ceria Yi Liu, Cun Wen, Yun Guo,* Xiaohui Liu, Jiawen Ren, Guanzhong Lu, and Yanqin Wang*[a]

CO disproportionation produces carbon deposits that cover form spectra indicate that the CO bond of the CO molecule active sites and induce catalyst deactivation. However, under- on Ce3+ ion is weakened. With CO bond dissociation, standing of this detrimental reaction on reduced ceria is asymmetrical inorganic carboxylate species are formed. These deficient. Herein, the reversibility and reaction mechanism of species are the key reaction intermediates in CO disproportio-

CO disproportionation on reduced ceria are investigated. The nation and are further converted to produce CO2. EPR reversibility of the CO disproportionation was studied by CO experiments indicate that the unpaired electrons produced by pulse, isotopic tracer, thermal analysis, and CO2 pulse the reduction weaken the CO bond through back-donation experiments. In situ diffuse-reflectance infrared Fourier trans- of electrons.

Introduction

Noble metal supported catalysts have been widely studied in Herein, our study into the reversibility and mechanism of CO many important reactions, such as the reverse –gas shift disproportionation is discussed. The reversible reaction of CO [1,2] reaction and CO2 reforming of methane. In these catalysts, disproportionation on reduced ceria was studied by the CO ceria is a widely employed support because of its unique pulse, isotopic oxygen tracer, thermal analysis (TG/DTA), and oxygen-storage capacity. The addition of ceria improves the CO2 pulse experiments. Moreover, in situ DRIFTS was conduct- performance of catalysts but these catalysts still suffer from ed after CO/CO2 treatment to study the reaction intermediates, severe deactivation. Catalyst deactivation can be attributed to and electron paramagnetic resonance (EPR) spectroscopy was several factors, such as over-reduction of ceria, formation of used to investigate the influence of the reduction on the prop- stable carbonate species, and formation of carbon deposits.[3–7] erties of the ceria. The reduction generated unpaired electrons, The carbon deposits usually arise from CO disproportiona- which weakened the CO bond in CO on absorbed Ce3+. tion,[1,8] which can be expressed as follows:

CO+COÐC+CO2 Based on this expression, the occurrence of CO disproportiona- Results and Discussion tion involves the dissociation of CO bonds. CO dissociation CO pulse experiments on noble metal-loaded rare earth oxide catalysts has been ex- tensively studied.[9–12] Mullins and Overbury[10] investigated the CO pulse experiments were conducted on reduced ceria adsorption and reaction of CO on Rh-loaded ceria by soft X-ray (R-CeO2; see Experimental Section) to study the forward photoelectron spectroscopy, and found that atomic carbon reaction of CO disproportionation. If disproportionation takes [11] place, CO can be detected in the pulse experiment. The was produced by CO dissociation on Rh/CeOx. Putna et al. 2 also detected CO dissociation on highly reduced Rh-loaded results of the CO pulse are shown in Figure 1. The intensity of ceria/YSZ (YSZ =yttria-stabilized cubic zirconia), but not on Pd- the CO signal increases gradually at first, and finally reaches [9] or Pt-loaded ceria/YSZ. Holmgren et al. suggested that CO equilibrium, whereas the intensity of the CO2 signal decreases disproportionation was the most reasonable explanation for with the increasing pulse number. The consumption of CO in the first several pulses is concomitant with the production of the CO2 formation on strongly reduced Pt/CeOx. Generally, the presence of a noble metal was considered a prerequisite for CO2. The CO consumption should not be attributed solely to CO disproportionation. adsorption because adsorption cannot generate CO2. Another To our knowledge, there have been two reports of CO potential reason for CO consumption was the reaction of the disproportionation on reduced ceria. Li et al.[13] detected car- adsorbed CO with hydroxy groups to form formate intermedi- bonate signals by in situ diffuse-reflectance infrared Fourier- [a] Y. Liu, C. Wen, Prof. Y. Guo, X. Liu, Dr. J. Ren, Prof. G. Lu, Prof. Y. Wang transform spectroscopy (DRIFTS) on H2-reduced ceria, and Lab for Advanced Materials, Research Institute of Industrial proposed that these carbonate species came from CO dispro- East China University of Science and Technology, Shanghai 200237 (P.R. portionation on the reduced ceria. Recently, Swanson et al.[14] China) detected carbon species, which were the products of CO Fax: (+ 86)21-64253824 E-mail: [email protected] disproportionation, on reduced ceria by in situ Raman spec- [email protected] troscopy. However, the reversibility and mechanism of this Supporting information for this article is available on the WWW under reaction have not, to date, been clarified. http://dx.doi.org/10.1002/cctc.200900246.

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These experiments were conducted by introducing isotopic oxygen into the recirculation volume. In our study, the catalyst,

CeO2, was more reducible than MgO. Moreover, the experi- mental conditions in our study are more favorable for oxygen exchange than those in the aforementioned studies.[1516] The CO pulse experiment was then performed on reduced ex-

changed ceria (RE-CeO2 ; see Experimental Section) to simulate

the CO pulse experiment on R-CeO2 and to identify the origin

of oxygen in the produced CO2. The results (Figure 2) show 16 16 18 that C O2 was the main product, and that little C O Owas 16 formed. The intensity of the C O2 signal was stronger than that of the C16O18O signal, which indicates that the 16O in the 16 C O2 was not from the sample lattice. If the CO had primarily

reacted with the lattice oxygen in RE-CeO2, the intensity of the Figure 1. MS signals of outlet gas during CO pulse experiment on R-CeO2 at 16 18 4008C. C O O signal should have been stronger than that of the 16 C O2 signal, as the aforementioned results showed that the amount of 18O in the sample lattice was more than that of 16O. ates, which further convert to CO2. However, this possibility Therefore, these results indicate that the production of carbon was ruled out because the formate intermediates are not de- dioxide results from CO disproportionation on reduced ceria. tected in the in situ DRIFTS experiment. Therefore, the evolved

CO2 may be from CO disproportionation or the reaction be- tween pulsed CO and lattice oxygen. To identify which reaction Thermogravimetric and differential thermal analyses was responsible for the CO2 formation, isotopic oxygen tracer To further verify the occurrence of the CO disproportionation, experiments were conducted. thermogravimetric and differential thermal analyses (TG/DTA) were carried out to establish the changes in weight and heat brought about by carbon deposit . TG/DTA experi-

Isotopic oxygen tracer experiments ments were performed on carbon-deposited ceria (D-CeO2 ; see Experimental Section) under oxidative conditions. If a carbon To analyze the 18O distribution in isotopic oxygen tracer deposit was present on the sample, combustion of the carbon experiment, the extent of oxygen exchange on the isotopic deposit would induce weight loss and exothermic phenomena. oxygen-exchanged ceria (E-CeO2 ; see Experimental Section) The TG/DTA curves on D-CeO2 under air are shown in was determined by the CO pulse experiment (see the Support- Figure 3. The TG plot shows two weight-loss steps. The weight ing Information, Figure A.2). The calculated ratio of the loss and endothermic peak before 2008C are attributed to amount of C16O18OtoC16O was about 3.3:1. The exchange 2 desorption of physically adsorbed water. Another steep extent of oxide catalysts has been studied in depth. Karasuda weight-loss step (ca. 0.08 %) occurred at about 6108C, and an and Aika conducted isotopic oxygen exchange (IOE) experi- exothermic peak was detected at the corresponding tempera- ments on irreducible MgO.[15] At 973 K, the rate of oxygen ex- ture in the DTA plot. The peaks at about 6108C can not be change was fast, and it reached equilibrium within 100 min. attributed to desorption of hydroxy or CO groups, because Royer et al.[16] indicated that IOE proceeded quickly on LaFeO . 3 these desorption processes are endothermic. Furthermore, The exchange reached equilibrium within 80 min at 812 K. these peaks can not be attributed to desorption of carbonate

Figure 2. MS signals of products during CO pulse experiment on RE-CeO2 at Figure 3. Thermogravimetric (TG) and differential thermal analysis (DTA)

4008C. plots on D-CeO2 under air flow.

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species either, according to the results of TG/DTA under N2 is roughly equal to 1:2. This ratio agrees well with the stoichio- atmospheres (see the Supporting Information, Figure A.3). No metric ratio of CO2 to CO in the CO disproportionation reaction obvious weight loss can be attributed to desorption of the car- formula, and this result further indicates that CO production is bonate species under N2 flow in the corresponding tempera- the result of the reverse CO disproportionation reaction on [17] ture range. However, previous studies have indicated that D-CeO2. oxidation of the carbon deposit takes place in this temperature range. Therefore, the weight loss and the exothermic peaks at In situ DRIFTS about 6108C can be attributed to the carbon deposit combustion. In situ DRIFTS experiments were conducted under the reactant

The production of CO2 and carbon deposit was detected by atmosphere to study the detailed reaction mechanism of CO CO pulse, isotopic oxygen tracer, and TG/DTA experiments, disproportionation (Figure 5 and Table 1). The spectra were all which verified the occurrence of CO disproportionation on R- recorded in the absence of gas-phase CO because the wave-

CeO2. However, this reaction is reversible, and the reverse reac- number of the IR band of gas phase CO is similar to that of tion, in which CO2 reacts with the carbon deposit to release the adsorbed CO. Thus, a spectrum was recorded after the CO, should also be verified on the carbon deposited ceria. sample was purged by He flow for 10 min (Figure 5a), in which

Herein, the CO2 pulse experiment was used to investigate the the signals S1 and S2 (Table 1) are observed. The signals at 1 reaction between pulsed CO2 and D-CeO2. about 2116 and 2173 cm (S1) are attributed to linearly

CO2 pulse experiments

In CO2 pulse experiments, if the reverse reaction of CO dispro- portionation were to take place on D-CeO2, pulsed CO2 would react with the carbon deposit to form CO. The results of the

CO2 pulse experiment are shown in Figure 4. The intensity of the CO2 signal increased as the pulse experiment proceeded. Simultaneously, the intensity of the CO signal decreased with the increasing pulse, and finally reached a constant level.

Figure 5. In situ DRIFT spectra recorded at 400 8C: a) The system was purged by He for 10 min after exposure to CO; b) the system was at equilibrium

under He flow after exposure to CO; c) the system had been treated by CO2 for 30 min. Inset: The signal in the range 2200–2000 cm1 under nine times magnification.

Table 1. Assignment of bands from in situ DRIFT spectra (Figure 5).

Signal n˜ [cm1] Species

3 + Figure 4. MS signals of outlet gas during CO2 pulse experiment on D-CeO2 S1 2116 CeCO CO linearly adsorbed on Ce at 4008C. 2173 CeCO CO linearly adsorbed on Ce4 +

Several reactions may occur in this experiment: CO adsorp- 2 1457, 1367 Unidentate carbonate tion, the reaction between CO2 and the reduced ceria, and the reverse CO disproportionation. The depletion of CO2 can not be due to CO2 adsorption because the adsorption does not S2 1540, 1303 asymmetrical inorganic carboxylate generate CO. Moreover, the CO2 depletion is not the result of oxidation of the sample by CO2 because no reaction occurred between R-CeO2 and CO2 during the CO2 pulse experiment on 1397 bridged carbonate

R-CeO2 (see the Supporting Information, Figure A.4). Therefore, the reverse CO disproportion must be responsible for CO production on D-CeO2. Moreover, based on the calculation, S3 1414 symmetrical inorganic carboxylate 18.6 mLCO2 is consumed and 35.6 mL CO is produced during the CO2 pulse. The ratio of CO2 consumption to CO production

ChemCatChem 0000, 00, 1 – 7 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemcatchem.org &3& These are not the final page numbers! ÞÞ Y. Guo, Y. Wang et al. adsorbed CO on Ce3+ and Ce4+ ions, respectively.[9,14,18] Com- about the steric effect, which regenerates the asymmetrical pared with the IR band of CO adsorbed on Ce4+ ions, that for inorganic carboxylate species. CO on Ce3+ ions is shifted to a lower wavenumber. This red shift indicates that CO bonds on Ce3+ ions are weakened. EPR spectroscopy The weakened CO bonds on Ce3+ ions are more easily disso- ciated, which facilitates CO disproportionation. The signals CO disproportionation is induced by the dissociation of CO labeled S2 (Figure 5, Table 1) are attributed to unidentate car- bonds. Electron back-donation has been proposed as the bonate species, bridged carbonate species, and asymmetrical reason for the weakening of CO bonds adsorbed on Ce3+ inorganic carboxylate species.[9] A spectrum was also recorded ions.[14] Furthermore, the role of electron back-donation in CO after the system reached equilibrium under He flow oxidation on perovskites has been studied by Rothenberg (Figure 5b), in which only signal S3 appears. This signal can be et al,[20] who suggested that the lack of electron back-donation assigned to the symmetrical inorganic carboxylate from Cr to CO is one of the factors that are responsible for the [19] species (Table 1). low CO oxidation activity of LaCrO3. Herein, this proposed A third spectrum (Figure 5c) was recorded during CO2 mechanism is studied by EPR spectroscopy, which detects contact with D-CeO2, which was used to investigate the inter- changes in the paramagnetic signals brought about by the mediates of the reverse CO disproportionation reaction, and reduction of ceria. EPR spectroscopy is widely used to study showed signals S1 and S2. The detection of signal S1 indicates unpaired electrons,[21–26] and the paramagnetic signals that CO2 takes part in the reverse CO disproportionation reac- generated by probe molecules, such as O2, are used to obtain tion, and CO is formed simultaneously. The regeneration of sig- detailed information on the catalyst.[22,25] nal S2 indicates that a key reaction intermediate is represented There are two types of signal in the EPR spectra (Figure 6), by this signal, because these intermediates should be present one at g= 1.962 and g= 1.948, and the other at g=2.016. in both the forward and reverse CO disproportionation reac- According to the previous reports.[22,27–29] the signal at g = tions. According to the assignments in Table 1, the unidentate 1.962 and g =1.948 can be attributed to electrons trapped by species and bridged carbonate species detected in signal S2 are bonded to the oxygen ions on the sample. If these carbon- ate species reacted with the carbon deposit to generate CO, the CO would be adsorbed on oxygen ions. This adsorption state is different from the initial adsorption state of CO, in which the CO molecules are linearly adsorbed on the cerium ions instead of on the oxygen ions. Therefore, the unidentate species and the bridged carbonate species are not the key reaction intermediates of the CO disproportionation. These [19] species are the products of CO2 adsorption on ceria. Howev- er, the asymmetrical inorganic carboxylate species are bonded to cerium ions, which indicates that these species can react with the carbon deposit to restore the initial adsorption state in CO disproportionation. Therefore, these asymmetrical inor- ganic carboxylate species are the reaction intermediates in both the forward and reverse disproportionation reactions. Figure 6. EPR spectra of fresh CeO2 treated by different numbers of 10% H2/Ar pulses. (a) 0; (b) 3; (c) 5; (d) 7; and (e) 10 pulses. When CO is adsorbed on R-CeO2, two adjacently adsorbed CO molecules couple together. Upon CO bond dissociation, the asymmetrical inorganic carboxylate species are produced, and 3+ are then desorbed to release CO2. During the reverse oxygen vacancies or Ce ions located at the CeO2 surface. disproportionation reaction, these asymmetrical inorganic However, some researchers considered that the Ce3+ signals carboxylate species can interact with the deposited carbon to were undetectable except at very low temperature (near liquid form CO. He temperature or below) because of its fast spin-lattice relax- Furthermore, the symmetrical inorganic carboxylate species ation, which would broaden the EPR lines.[23,26] Therefore, in (signal S3; Table 1) should arise from the transformation of the this study, the signal at g=1.962 and g= 1.948 is attributed to asymmetrical inorganic carboxylate species. This transforma- electrons trapped by oxygen vacancies. The intensity of the tion is caused by the change of the chemical environment signal at g= 1.962 and g =1.948 does not change with the around the carboxylate species. Purging with helium removes increased number of H2 pulses. The signal at g=2.016 is attrib- [25,29] most of the adsorbed carbonate species (Figure 5b), and the uted to O2 species. With increasing number of H2 pulses steric effect produced by these carbonate species disappears. (Figure 6a!e), the intensity of the signal at g =2.016 also in- Thereafter, the asymmetrical inorganic carboxylate species creases. The stronger signal intensity at g =2.016 indicates that were converted to the symmetrical inorganic carboxylate more O2 species are produced. This phenomenon suggests species. When the catalyst was exposed to CO2, the formed that the number of unpaired electrons increases with an unidentate species and bridged carbonate species brought increasing degree of reduction of ceria.

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In the reduction process, more and more unpaired electrons DRIFTS experiments verified the mechanism of CO dispropor- are created by the breakage of CeO bonds, and these elec- tionation. In the CO disproportionation reaction, two adjacent trons are trapped by the oxygen vacancies. On exposure to air, adsorbed CO molecules coupled together. The asymmetrical [30] the oxygen vacancies in the sample adsorb O2 molecules. inorganic carboxylate species, which were the reaction inter- The unpaired electrons that are trapped in oxygen vacancies mediates, were formed by CO bond dissociation. Further- can then transfer to O2 molecules to form paramagnetic O2 more, EPR experiments showed that the unpaired electrons, [25] species. The formation of O2 species indicates that the un- which were produced by the reduction, were back-donated to paired electrons are transferable. Because most of the unpaired the adsorbed CO on Ce3+ ions. This back-donation weakened electrons are transferred to O2 molecules, the amount of re- the CO bonds, and facilitated CO disproportionation. The re- sidual unpaired electrons that are still trapped in the oxygen sults in this study suggest that adjusting the electronic proper- vacancies, is very low, which is why the paramagnetic signal at ties of ceria may be useful in hindering CO disproportionation. g= 1.962 and g= 1.948 does not increase with the reduction of ceria. Thus, the unpaired electrons which weaken the CO bonds are produced by the reduction. These unpaired Experimental Section electrons are back-donated to the CO bonds of adsorbed CO Synthesis on Ce3+ ions, which induces the CO disproportionation on The ceria was prepared by the hydrothermal synthesis method.[31] R-CeO2. The raw materials, [(NH4)2Ce(NO3)6] (18.24 mmol) and a stoichio- The mechanism of the CO disproportionation on R-CeO2 is metric amount of urea, were dissolved in deionized water (80 mL). proposed and shown in Scheme 1. After exposure to a CO The solution was then transferred into a Teflon-lined container, and atmosphere, the sample adsorbs CO molecules on Ce3+ and hydrothermally treated at 1308C for 4 h with stirring. After cooling Ce4+ ions. The CO bonds on Ce3+ ions are weakened by to room temperature, the precipitate was removed by filtration, back-donation of the unpaired electrons (Scheme 1a). When washed with deionized water (3 30 mL), and then dried at 1108C. two adjacent adsorbed CO molecules couple together, the After calcination at 5508C for 4 h, a pale yellow powder was obtained and defined as fresh CeO (F-CeO ). weakened CO bond is dissociated and an asymmetrical inor- 2 2 ganic carboxylate intermediate is formed (Scheme 1b). Further The reduced CeO2 (R-CeO2) was prepared by treating F-CeO2 under conversion of these intermediates generates CO2. The re- H2 flow at 6008C for 3 h. Samples were stored at room tempera- ture under He flow (BOC Gas, purity 99.999%) for further study. adsorption of CO2 on the catalyst surface forms unidentate and bridged carbonate species and, owing to the steric effect In this study, the gases were dehydrated by passing through a desiccator containing a mixture of molecular sieves and silica gel. brought about by the carbonate species, carboxylate species. When the adsorbed carbonate species are removed, the asym- metrical inorganic carboxylate intermediates transform to sym- Characterization metrical inorganic carboxylate species (Scheme 1c) because CO pulse experiment: In situ-reduced CeO (50 mg) was purged by the steric effect diminishes with the desorption of carbonate 2 He flow (30 mLmin1) at 4008C. The loop volume was 73.7 mL, and species. In the reverse CO disproportionation reaction, the car- 35.7% CO/Ar was pulsed into the system at intervals of 30 s. Argon bonate species that are formed by the CO2 adsorption bring was used as the calibration diluent. The outlet gas components about the steric effect. Therefore, the asymmetrical inorganic were monitored by an online Quadrupole Mass Spectrometer carboxylate intermediates are regenerated and further react (INFICON Co. Ltd., IPC400). with the carbon deposit to form CO. Isotopic oxygen-exchange (IOE) experiment: IOE was conducted in 16 a quartz tube reactor. After pretreatment with O2 at 5508C for

3 h, F-CeO2 (50 mg) was treated under Ar flow at this temperature Conclusions for 30 min. The system was then cooled to room temperature 18 under Ar flow, and 10% O2/He was pulsed into the system. The In this study, both the forward and reverse CO disproportiona- 16 sample was subsequently reoxidized with O2 at 5508C for 30 min, tion on reduced ceria were studied by the experiments of CO and purged by Ar flow at this temperature for 30 min. After the pulse, isotopic oxygen tracer, TG/DTA, and CO2 pulse. In situ system had cooled to the designated temperature under Ar flow,

Scheme 1. A schematic mechanism of the CO disproportionation on R-CeO2. S1–3 indicate the corresponding bands in the in situ DRIFT spectra (Figure 5 and Table 1).

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18 10% O2/He was pulsed into the system. This procedure was re- 20673037), the New Century Excellent Talents in University 18 18 peated until the O2 conversion had reached 100%. The O2 gas (NCET-05-415), and the Commission of Science and Technology of was purchased from the Shanghai Research Institute of Chemical Shanghai Municipality (05QMX1415), China. Industry (97% purity). The effluent gas was monitored by online MS. The temperature for IOE treatment was selected at 6008C (see the Supporting Information, Figure A.1). Isotopic oxygen- Keywords: back-donation · cerium · heterogeneous catalysis · exchanged ceria (E-CeO2) was prepared by treating F-CeO2 under IR spectroscopy · reaction mechanisms 18 O2 at this temperature for 2 h. In order to determine the ratio of 18 16 Oto O in the E-CeO2 lattice, 35.7% CO/Ar was pulsed onto the sample at 4008C, and the effluent gas was monitored by the [1] A. Goguet, F. Meunier, J. P. Breen, R. Burch, M. I. Petch, A. Faur Ghenciu, online MS. J. Catal. 2004, 226, 382 –392. [2] J. B. Wang, Y.-L. Tai, W.-P. Dow, T.-J. Huang, Appl. Catal. A 2001, 218,69– Isotopic oxygen tracer experiment: CO was pulsed onto reduced 79. E-CeO2 (RE-CeO2), which was obtained by reducing E-CeO2 under [3] J. M. Zalc, V. Sokolovskii, D. G. Lçffler, J. Catal. 2002, 206, 169– 171. H2 flow at 6008C for 3 h. After the system had cooled to [4] X. Wang, R. J. Gorte, J. P. Wagner, J. Catal. 2002, 212, 225– 230. 4008C, 35.7% CO/Ar was pulsed into the system. The effluent gas [5] X. S. Liu, W. Ruettinger, X. M. Xu, R. Farrauto, Appl. Catal. B 2005, 56, was monitored by the online MS. 69–75. [6] M. M. V. M. Souza, D. A. G. Aranda, M. Schmal, Ind. Eng. Chem. Res. 2002, Thermogravimetric and differential thermal analyses (TG/DTA): To 41, 4681 –4685. obtain a detectable amount of carbon deposit from CO dispropor- [7] J. Beckers, C. Gaudillere, D. Farrusseng, G. Rothenberg, Green Chem. tionation, R-CeO2 was treated under CO flow at 4008C for 3 h. 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[23] Catalytic Science Series: Catalysis by Ceria and Related Materials (Ed.: A. Trovarelli), Imperial College Press, London, 2002, pp. 169–171. EPR spectroscopy: Spectra were recorded at room temperature on [24] M. W. Zhao, M. Q. Shen, J. Wang, J. Catal. 2007, 248, 258 –267. a Bruker EMX-8/2.7 EPR spectrometer. F-CeO (0.1 g) was put into 2 [25] E. Abi-aad, R. Bechara, J. Grimblot, A. Aboukas, Chem. Mater. 1993, 5, the self-designed quartz tube reactor, and pretreated at 5508C 793– 797. under He flow for 1 h. Then 10% H2/Ar was pulsed into the system [26] C. Oliva, G. Termignone, F. P. Vatti, L. Forni, A. V. Vishniakov, J. Mater. Sci. at 4008C. After it had cooled to room temperature, the sample 1996, 31, 6333 –6338. was exposed to air for 1 h, and the EPR spectra were recorded at [27] J. Soria, J. C. Conesa, A. Martnez-Arias, Colloid Surf. A 1999, 158, 67– 74. the X-band frequency (ca. 9.8 GHz). The signals were calibrated [28] A. Martnez-Arias, M. Fernndez-Garca, C. Belver, J. C. Conesa, J. Soria, with 2,2-diphenyl-1-picrylhydrazyl (g=2.0036). Catal. Lett. 2000, 65, 197– 204. [29] A. Martnez-Arias, R. Cataluna, J. C. Conesa, J. Soria, J. Phys. Chem. B 1998, 102, 809 –817. Acknowledgements [30] C. T. Campbell, C. H. F. Peden, Science 2005, 309, 713 –714. [31] R. Si, Y. W. Zhang, S. J. Li, B. X. Lin, C. H. Yan, J. Phys. Chem. B 2004, 108, 12481 –12488. This project was supported financially by the National Basic Research Program of China (No. 2004CB719500), the National Received: September 25, 2009 Natural Science Foundation of China (No. 20601008 and Published online on && &&, 2009

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Y. Liu, C. Wen, Y. Guo,* X. Liu, J. Ren, G. Lu, Y. Wang* && – &&

Mechanism of CO Disproportionation on Reduced Ceria Disproportionate representation: In are formed. The unpaired electrons, CO disproportionation on reduced ceria, which are produced by reduction of the adjacent adsorbed CO molecules couple ceria surface, weaken the CO bond together. Upon CO bond dissociation, adsorbed onto Ce3+ through back- asymmetrical inorganic carboxylate donation of electrons. species, the key reaction intermediates,

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